This invention relates to a communication system in which broadband data signals are transported across a synchronous digital hierarchy network (SDH).
The choice of digital transmission bearers within telecommunication networks has widened over the past decade with the emergence of new standards. The Plesiochronous Digital Hierarchy (PDH) which offered standard interface rates such as 1.544 Mbit/s, 2.048 Mbit/s, 34.368 Mbit/s and 139.468 Mbit/s has now been superseded by the Synchronous Digital Hierarchy (SDH). SDH has been designed to carry signals with PDH interface rates by xe2x80x98mappingxe2x80x99 them into virtual containers and, in fact, the majority of interfaces on contemporary SDH equipment are at PDH rates. This xe2x80x98mappingxe2x80x99 feature also allows for future signal types to be transported across SDH networks in parallel with legacy signal types. Provision has also been made within SDH for increasing bandwidths of signals while remaining compatible with existing equipment.
Both PDH and SDH systems fall into the broad category called Time Division Multiplex (TDM) equipment as they have been optimized to carry signals which have a constant, uninterrupted stream of data bits. As a consequence of this optimization, traditional voice traffic can be transported at a variety of capacity levels with reasonable efficiency. In contrast data-type signals, which are predominantly packet based, suffer from transport inefficiencies when carried over SDH and PDH systems. A major cause of this inefficiency is when a suitable container size does not exist for the bandwidth of the chosen packet rate.
Inverse multiplexing is a method for transporting non-standard or higher bit-rate signals over existing lower bit-rate communications channels or bearers. By carefully matching the number of lower-rate channels used for inverse multiplexing with the required packet rate an efficient data transport system can be constructed using TDM equipment. Inverse multiplexing is particularly suited to the handling of Asynchronous Transfer Mode (ATM) traffic, and a method has been devised (called IMA) for inverse multiplexing ATM cells into low capacity PDH signals by breaking up and re-constructing the broadband ATM signal on a cell-by-cell basis. Subsequently, these PDH signals may be mapped into an SDH transport system resulting in a 2-stage process with unnecessary signal processing whenever IMA is to be carried over SDH. Furthermore, it is difficult to apply the IMA method to other types of broadband data signal since the control and communication mechanism used by IMA relies on the presence of ATM cells.
The present invention seeks to provide an improved communications by utilizing the Inverse Multiplex method in a manner which is more suitable and more efficient for SDH transport systems and which is compatible with existing SDH equipment.
According to this invention a communications system having an SDH path between two nodes in which the path has virtual containers of a predetermined bandwidth, including at one node, means for receiving data including packet based data having a bandwidth greater than said predetermined bandwidth, and means for inverse multiplexing said data in a byte format regardless of packet boundaries onto a plurality of virtual containers for transmission to said other node; means at the other node for receiving and reassembling said data; and means for compensating for delays caused by the different path lengths of individual virtual containers.
The communications system is suitable for a wide range of broadband data types and can compensate for the specific timing impairments introduced by the SDH transport system.
As is known, a virtual container is a logical entity that exists only in a Synchronous Transport Module (STM), and contains both overhead information and pay load data. Different types of virtual containers have been defined by International Standards bodies to incorporate: the size and structure of each container; the type of payload data and method for encoding the timing of data; use of overhead bytes for management and maintenance purposes. Current virtual container (VC-n) types include VC-12, VC-2, VC-3 and VC-4. This invention proposes a method for
using existing virtual containers to transport a new inverse multiplex mapping denoted as VC-IM-n. The use of byte mapping enables the original digital content of the signal and its phase/frequency properties to be accurately preserved without the excessive use of bandwidth which would be required by an ATM cell based system.